Proof by Erasure

From 1930 to 1933 von Neumann often lectured to college groups in Princeton and enjoyed it immensely. It inspired him and he usually spoke without notes. He was said to be an indifferent teacher. His lectures were popular, though he was not easy to follow because he talked very rapidly. Stories are told of his lecturing while writing on the blackboard and erasing what he had written so quickly that students never got a chance to see it. In one of these lectures a colleague made an often quoted remark. He waited until John terminated his proof and then said:

I see. Proof by erasure.